Restore Codecov carryforward after the main rebaseline (carryforward reset, step 2)

[mengw15]

Task Summary

Step 2 (final) of the Codecov carryforward reset (step 1: #6850). Once main
has a clean full-build baseline under the current flag set, re-enable
flag_management.default_rules.carryforward: true so single-stack PRs keep
backfilling non-run flags from the base report. Net config is identical to before
the reset — the only effect is rebaselining Codecov's carryforward data on main.
Same reset previously done in #6080 → #6088.

Task Type

• Refactor / Cleanup
• DevOps / Deployment / CI
• Testing / QA
• Documentation
• Performance
• Other

[github-actions]

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